[libcxx] Improve tests to use the UNSUPPORTED lit directive

Quite a few libcxx tests seem to follow the format:
 #if _LIBCPP_STD_VER > X
   // Do test.
 #else
   // Empty test.
 #endif
We should instead use the UNSUPPORTED lit directive to exclude the test on
earlier C++ standards. This gives us a more accurate number of test passes
for those standards and avoids unnecessary conflicts with other lit
directives on the same tests.

Reviewers: bcraig, ericwf, mclow.lists

Differential revision: http://reviews.llvm.org/D20730

llvm-svn: 271108

libcxx/test/std/experimental/optional/optional.object/optional_void.fail.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
 // <optional>
 
 // T shall be an object type and shall satisfy the requirements of Destructible
@@ -15,11 +16,7 @@
 
 int main()
 {
-#if _LIBCPP_STD_VER > 11
     using std::experimental::optional;
 
     optional<void> opt;
-#else
-#error
-#endif  // _LIBCPP_STD_VER > 11
 }